With the continued emphasis on highly integrated electronic devices, there is an ongoing need for semiconductor memory devices that operate at higher speeds and lower power and that have increased device density. To accomplish this, devices with aggressive scaling and multiple-layered devices with transistor cells arranged in horizontal and vertical arrays have been under development.
In one approach, planar memory cells, for example NAND memory cells, are formed in a conventional horizontal array. Multiple horizontal arrays are then stacked in a vertical direction. Limitations associated with this approach include poor reliability in the resulting device, since critical lithography steps are required for each layer in achieving the minimum feature size. In addition, in this configuration, the size of the driver transistors for driving the control gates is a function of the number of layers; therefore, the driver transistors are scaled as a multiple of the number of layers. This can lead to integration issues and heat removal concerns.
In another approach, multiple-layered memory devices with vertically oriented channels have been under development. In one configuration, a plurality of gate layers are formed on a substrate, and a vertical channel penetrates the plurality of gate layers. In each vertical channel, a lower gate layer is configured to operate as a lower select gate, a plurality of middle gate layers are configured to operate as control gates, and an upper gate layer is configured to operate as an upper select gate. The control gates can include charge storage layers adjacent the vertical channel, so the devices can operate as non-volatile memory devices. Upper select gates neighboring each other in a first horizontal direction are connected to operate as row select lines for the device. Vertical channels neighboring each other are connected in a second horizontal direction to operate as bit lines for the device.
Others attempting the vertically oriented channel approach have met with limited success. In one configuration, a bottom portion of the vertical channel is connected to a common source diffusion layer that is formed in the substrate. The common source diffusion layer is doped to have an n+ doping, and the underlying substrate has a p type doping. Accordingly, a p-n junction is formed between the common source diffusion layer and the underlying substrate. The vertical channel is isolated by the n+ region in the substrate; Therefore, it is difficult to control the potential of the vertical channel through any electrode and it is necessary to apply a negative voltage level on the control gates to erase the memory cells. Such a negative voltage level can require a more complicated device power supply circuit, increasing device cost. Further, application of a negative voltage level is inconsistent with the power arrangement of conventional NAND memory devices, hindering straightforward replacement of conventional NAND devices by the vertically oriented channel memory devices. Furthermore, when an erase operation takes place, the erase operation requiring the injection of holes into the charge storage layers of the control gates adjacent the vertical channel, any holes in the vertical channel are depleted by the injection into the charge storage layers. With the depletion of holes, the potential of the vertical channel is reduced, which makes the erase operation less effective in time.
In order to prevent hole depletion in the vertical channels, hole injection by a gate-induced-drain-leakage (GIDL) operation has been proposed, in order to maintain the potential of the vertical channels at a suitable level. However, vertical channel potential control through GIDL effect is not straightforward or can be unstable compared with direct body biasing, because it can be affected easily by the junction profiles, which can result in the erased Vth distribution degradation. In addition, the GIDL effect is likely to incorporate hot hole injection into the lower select transistor or edge cell in the vertical channel strings, which can deteriorate endurance reliability characteristics of the resulting device.